The field of the invention is electron beam lithography, in particular, low-aberration magnetic lenses.
A uniform magnetic field (solenoid field) oriented along the electron beam axis is the simplest of electron lenses and has been employed in various electron beam systems. Electrons radiating from a point object execute, by virtue of their transverse velocity component, one cyclotron orbit in the transverse plane, returning to the optic axis. Thus, an image is formed with unity magnification. A major advantage of the solenoid lens is that there is no prescribed optic axis, hence a shift (deflection) of the beam by a transverse field will cause the beam to shift position, but maintain the same focal plane. A major disadvantage of these lenses is that they produce no demagnification of the object, so that defects in the source (reticle, shaping aperture) are reproduced in the image.
The classical solenoid field periodic focusing is depicted in FIGS. 5A and 5B. FIG. 5A shows a cross section of solenoid 5, with the drawing convention that the wires in the coil are denoted by xe2x80x9cXxe2x80x9d s 10 and 12, one of the sides being wires carrying current forward out of the plane of the paper to the viewer and the other being wires carrying current out of the plane of the paper away from the viewer. The uniform solenoid field 23 of solenoid 5 has no predefined optic axis and periodically focuses the beam with one to one magnification, as shown in the beam trace plot in FIG. 5B, in which beam trace 120 comes to a focus at regular intervals with a periodicity determined by the beam energy (acceleration potential) and the strength of the solenoid field.
The invention relates to a magnetic lens employing a solenoid field and having substantial demagnification.
A feature of the invention is the introduction of passive magnetic pole pieces in the solenoid field in such a way as to produce high demagnification of the object with very low geometric aberrations.